Application of Sunwoele HFV100 High Voltage VFD (Variable-frequency Drive) in Main and Auxiliary Equipment of Thermal Power Plant 300MW Unit

Abstract: This paper proposes a complete system solution for the application of high-voltage frequency conversion transformation of main and auxiliary equipment of thermal power plant 300MW units. The problems existing in the transformation of the power system and the good economic benefits obtained by the variable frequency speed regulation technology are analyzed and demonstrated.

I. Introduction

A thermal power plant is an energy conversion factory. It converts primary energy such as coal and oil, chemical energy, into electrical energy, a secondary energy with wide versatility and high efficiency. The product (electrical energy) cannot be stored, and the power generation, power supply and electricity consumption are completed at the same time, and it requires high speed and high quality.

The thermal power generating set is composed of three main engines, boiler, steam turbine and generator, and many auxiliary equipments. The task of the boiler is to produce steam, that is, to convert the chemical energy of coal, oil and other fuels into thermal energy of steam with a certain pressure and temperature; the steam turbine converts the thermal energy of the steam into mechanical energy; and the generator converts the mechanical energy into the final product of the power plant. - Electricity.

The power industry is an important application field of high-voltage VFD (Variable-frequency Drive) products. The research on the applicability of frequency conversion technology for the main and auxiliary equipment of power industry units shows that there are seven types of equipment in each unit that can realize frequency conversion applications. Each unit unit needs to be equipped with 13 VFD (Variable-frequency Drive) units. According to statistics, the fans and pumps used in China's thermal power plants have a total capacity of 15,000 MW of fans and pumps, including fans, induced draft fans, feed pumps, circulating pumps, and mortar pumps. 5% to 8%.

At present, the fans and pumps of thermal power plants in China are basically driven by fixed speed. The fixed-speed-driven fan adopts the inlet damper, and the water pump adopts the outlet valve to adjust the flow, which has serious throttling loss. Especially in the variable load operation of the unit, the operation efficiency is reduced because the operation of the fan and the water pump deviates from the high efficiency point. The existing method of adjusting the flow does not change the speed of the motor, so the power consumed by the motor remains unchanged, and the pressure of the pipe network is too large, which is not conducive to the safe and stable operation of the pipe network. If VFD (Variable-frequency Drive) is used to adjust the speed of the motor to achieve the flow rate expected by the user, a lot of electric energy can be saved. The effective speed regulation operation of the auxiliary motor of the power plant is directly related to the benefit of the power plant.

2. Frequency conversion transformation and process introduction of main and auxiliary equipment of power unit

1. Overview of Boiler Fan Frequency Conversion Transformation

The air volume and air pressure margin of the power plant boiler fan and the peak-shaving operation of the unit cause the operating point of the fan to deviate from the design high-efficiency point, which greatly reduces the operating efficiency of the fan. According to statistics, under normal circumstances, when the operating point and the design point of the fan with the air volume adjusted by the damper deviate by 10%, the fan efficiency will drop by about 8%; when it deviates by 20%, the fan efficiency will drop by about 20%; At 30%, the fan efficiency will drop by more than 30%, which is an unavoidable loss for fans that use imported dampers to adjust the air volume. It can be seen that a large part of the electricity consumption of the induced draft fan is consumed by the regulating door. Therefore, improving the adjustment method of the fan is the most effective way to improve the operating efficiency of the fan and reduce the power consumption of the fan. If the high-voltage VFD (Variable-frequency Drive), which has been widely used in China, is installed on the fan, the speed of the fan motor is controlled, so as to realize the adjustment of the air volume to meet the change of the boiler load, so that the air valve in the adjustment of the air door can be adjusted. Energy losses are saved.

2. Boiler fan process introduction

2.1 Induced fan

The induced draft fan is an important auxiliary machine in the boiler production process of the thermal power plant. The medium conveyed by the induced draft fan is flue gas, and the maximum temperature generally should not exceed 250 degrees. The boiler structure is complex, and there are dust removal and desulfurization equipment for the flue gas. The flue gas resistance is relatively large. The flue gas can be removed by using the induced draft fan to exhaust the smoke. At the same time, the induced draft fan is also an important equipment to ensure the negative pressure generated by the boiler combustion chamber.

2.2 Blower

In order to satisfy the complete combustion of coal, the air volume required for the combustion of pulverized coal is supplied through the blower (including the primary and secondary fans). The primary air enters the air chamber at the bottom of the furnace, and an air duct igniter is connected in parallel on the primary air duct; the secondary air directly enters the furnace in two layers through the secondary air box on the upper part of the furnace. Adjustment baffles are provided throughout the flue gas system to allow for adjustment control during operation and during start and stop of the furnace.

2.3 Powder Discharger

The pulverized coal is heated by the hot air and sent to the boiler through the pulverizer. The pulverizer is the power source for the flow of the gas-powder mixture in the pulverizing system. It overcomes the resistance in the flow process and completes the pneumatic conveying of the pulverized coal. In the direct-blown pulverizing system and the intermediate storage silo-type spent gas pulverizing system, the pulverizer also acts as a primary fan, and the pulverized coal airflow is blown to the furnace by the pressure generated by it. The powder discharge machine of the pulverizing system is adjusted by the inlet baffle, and in order to reduce the tertiary air volume, the inlet pipe of the powder discharge machine adjusts the air volume by adjusting the small baffle, which wastes a lot of electric energy. To reduce energy consumption,

The powder discharge fan is installed in the ball mill in the pulverizing system, after the coarse powder separator and the fine powder separator, to ensure that the raw coal is mixed with the hot air from the air preheater and the recirculating air from the outlet of the powder discharge machine in the ball mill to dry the raw coal. , and grinded into pulverized coal, and the negative pressure airflow generated by the exhaust fan, the fine powder separator separates the air and the powder, and the pulverized coal falls into the pulverized coal silo for storage. The remaining gas contains 5-10% of The air and powder mixture is sent into the furnace as tertiary air through the outlet of the powder exhaust fan or into the primary air box as the primary air mixed with the pulverized coal dropped by the powder feeder and sent into the furnace. From the above process introduction, we can see that in the pulverizing system, the pulverizer mainly provides negative pressure for the pulverizing system. After the frequency conversion of the powder discharge machine is carried out, the inlet air door of the powder discharge machine is fully opened, and the outlet air door is properly adjusted at the same time, which can fully meet the needs of providing negative pressure for the pulverizing system without affecting the tertiary air.

3. Introduction of Boiler Related Pump Technology

3.1 Boiler feed pump

The task of the feed pump is to push the feed water with a certain temperature in the water storage tank of the deaerator, and generate enough pressure through the feed pump to drive it into the steam drum. After the steam is heated, it enters the steam turbine to do work, and drives the generator set to generate electricity. According to the characteristics of electric energy production and the special requirements of thermal boiler operation, the feed pump must run continuously. This is not only related to normal power generation, but also directly related to the safety of boiler equipment. Therefore, the boiler feed water pump is the most important water pump in the power plant, known as the heart of the generator set. The process flow of the feed pump is shown in Figure 1 below:

At present, most of the boiler feed pumps on site use valves to adjust the flow, pressure and other parameters to meet the needs of the feed water required for boiler operation. Under this adjustment method, the system mainly has several problems:

1) The feed pump is operated at a constant speed, the valve adjustment and throttling loss are large, the outlet pressure is high, the pipe damage is serious, and the system efficiency is low, resulting in a waste of energy.

2) When the flow rate decreases and the valve position opening decreases, the pressure difference between the front and rear of the adjustment valve increases, the work safety characteristics become worse, the pressure loss is serious, and the energy consumption increases.

3) Long-term 40-70% valve opening will accelerate the wear of the valve body itself, resulting in poor valve control characteristics.

4) The high pressure of the pipe network threatens the sealing performance of the system equipment, and in severe cases, the valve leaks and cannot be closed tightly.

5) The equipment has a short service life, a large amount of daily maintenance, and high maintenance costs, resulting in a great waste of various resources.

One of the important means to solve the above problems is to use the frequency conversion speed control technology. The high-voltage VFD (Variable-frequency Drive) is used to control the frequency conversion of the motor of the feed water pump to realize the variable load regulation of the water flow. In this way, it not only solves the shortcomings of poor control valve regulation, such as poor regulation linearity and large pure delay, but also improves the reliability of system operation; more importantly, it reduces the pressure flow loss caused by the change of the regulating valve orifice. It reduces the wear of the control valve, reduces the damage of the system to the sealing performance of the pipeline, prolongs the service life of the equipment, reduces the maintenance amount, improves the economy of the system, saves energy, and provides a good solution for reducing the power consumption rate of the power plant. way.

3.2 Boiler condensate pump

Most of the steam in the exhaust gas after the steam turbine has done work is condensed into water by the circulating cooling water of the condenser, and the condensed water enters the hot well and is pumped out by the condensing pump (two-use and one-standby), after passing through the shaft seal cooler and the water level automatic adjustment device, part of it passes through the low pressure. After the heater is heated, it is sent to the deaerator; the other part is returned to the hot well through the recirculation regulating valve to ensure the liquid level of the hot well. The process flow diagram of the condensate pump is shown in Figure 2 below:

At present, the configuration of the condensate pump in the power plant unit is basically one for use and one for standby. After a certain pump has been running for a period of time, the pump must be reversed frequently, and the water pressure and flow at the outlet of the condensate pump are adjusted by valves. The main problems of the system are:

1) The coagulation pump is operated at a constant speed, the valve adjustment and throttling loss are large, the outlet pressure is high, the pipe damage is serious, and the system efficiency is low, resulting in a waste of energy.

2) When the flow rate decreases and the valve position opening decreases, the pressure difference between the front and rear of the adjustment valve increases, the work safety characteristics become worse, the pressure loss is serious, and the energy consumption increases.

3) Long-term 40-70% valve opening will accelerate the wear of the valve body itself, resulting in poor valve control characteristics.

4) The high pressure of the pipe network threatens the sealing performance of the system equipment, and in severe cases, the valve leaks and cannot be closed tightly.

5) Because the pump is often reversed and the pump is often erected, the pipe network is greatly impacted, the service life of the related equipment is short, the daily maintenance is large, and the maintenance cost is high, resulting in a great waste of various resources.